Dave sorts out vehicle-mounted antenna SWR issues

Photo by Katie Musial.

Many thanks to Dave (K1SWL) who writes:


Comments on vehicle-mounted antennas

by Dave Benson (K1SWL)

As with Rand’s recent post about his effective vehicle setup, I and others also use a small operating table inside the vehicle.  I’ve tried a number of approaches to antennas.  Without elaborating on those schemes, I’ll note that winter is now closing in here in NH. As a result, I’m now operating exclusively from my truck. My interest is now in minimizing setup and tear-down times.  Barry (WD4MSM)

also commented about the improvement in vehicle-mounted antennas with an added ground.  I’d like to quantify that.

I’d recently ordered a number of Hustler Mobile antenna components. They’re used as a stationary-portable setup using that company’s high-quality mag-mount. As I first evaluated the antenna, I was disappointed to find the minimum SWRs to be on the high side.

These results were related to the ‘floating’ coax shield, which serves as a counterpoise with the mag-mount setup.  Worse yet, these results were inconsistent. Touching the coax connector shell at the antenna analyzer caused the SWR to jump up, as did just changing the way I held the analyzer. Bad juju! It means RF inside the vehicle, with the potential for RF-‘hot’ symptoms at the rig..  Adding a 1:1 balun inline eliminated the stray RF at the rig, but didn’t do much for the SWR. It’s also just one more gadget to bring along.

A better fix was a custom bracket that bolted to the truck frame. I first confirmed that there was low-resistance continuity between a target location and the vehicle’s cigarette lighter shell.  This was something of a ‘comedy of errors’. I had a sheet-metal angle bracket on hand and went to work enlarging a hole in it. This had the usual outcome: a drill bit grabbed the workpiece and spun it. The bracket itself was buckled beyond redemption and my finger’s now healing well.  A length of 1-1/2 inch aluminum angle bracket was just the ticket.   Note that the mounting hole needs to be offset from the coax fitting mount. This avoids an interference between the mounting bolt and coax connector shell. Ask me how I know.  The bracket assembly uses a specialty coax fitting from DX Engineering. It’s their part number DXE-UHF-FDFB.

This bracket is bolted down on one of the corners of the Tacoma’s passenger seat assemblies.  It’s the closest location to the antenna I found without drilling holes and cutting the coax.  For this vehicle, it’s a 10mm bolt and was paint-coated for appearance reasons. I replaced it with a stainless-steel bolt from a hardware store. It’s important to include a split-lockwasher between the bolt and the bracket. This’ll keep the conductivity to the frame good over time. The bracket is deburred and its corners rounded to preclude injury to passengers.

In any event, it’s out of the way of the seat’s legroom space. A 3-foot coax cable assembly brings the coax nicely up behind the rig atop the operating surface.

So- how’d it work? It’s like the difference between night and day!  The broad SWR curves vanished – replaced by typical characteristics for monoband antennas. The sensitivity to handling the coax has vanished.  (A representative curve at right.) The curves are narrower, and that’s actually a good sign- it means that unwanted resistances have been reduced. 

With this fix in place, here are the SWR minima:

Frequency    SWR

14060        1.04:1

21060        1.05:1

28060        1.16:1

I took advantage of the CQ Worldwide CW Contest this past weekend. I was able to work 101 stations on 10M, 15M and 20M with this setup.  That was from a State Park 5 minutes away.  The attraction was a large and sunny parking lot, and solar gain was such that I needed to leave the truck door open several times.  This area is kept plowed out in winter, and I may try for the POTA ‘kilo’ award from there at the 1000-contact benchmark.  

We’ll see….   73, K1SWL

7 thoughts on “Dave sorts out vehicle-mounted antenna SWR issues”

  1. Nice work. I use a hatch mount on my SUV and I stripped away some of the coax covering about a foot from the antenna to solder a ground wire and attach it to a convenient bolt on the hatch (using a star washer for assured grounding.) This gave me similar results. I considered adding another ground strap from the hatch to the car body but it wasn’t necessary. I operate Mobil QRP CW on a daily basis and have no problem making contacts.

  2. Excellent idea. A vehicle chassis makes quite a good ground. I’m not sure if it’s the two tons of steel or the capacitance to ground that makes it work, but if it works, it works.

    My mobile antenna (home brewed) is mounted on my truck’s hitch. The hitch receiver is a good chassis connection point. It has worked well for several POTA activations on 20m and 40m.

  3. For a bracket DXEngineering sells tower brackets for RF connectors. They have holes for feed thru connectors and holes for U-clamps to mount on a tower leg. This bracket could be used for the mobile connecting bracket if one cannot find or make on their own.

    I have a couple on my tower for bringing down my HF antenna feed lines. It would help drain away the energy in a lightning strike since most of the energy is on the feed line shield.

    73, ron, n9ee

  4. gang-

    Thanks for the comments! The sheet-metal angle bracket that I’d mangled was out of stock when I sent the writeup to Tom. Now, apparently- so is that cool coax feedthrough connector.

    However you can do it, a good ground to the the vehicle frame really makes a difference. There’s a lot to be said for a 2-ton counterpoise!

    73- K1SWL

  5. Dave, I’m curious as to how you’re powering the radio when operating stationary mobile: car battery or a battery dedicated just to the radio?

    I use a separate LiFePO4 battery, whether I’m QRP or at 100 watts. This has the unfortunate effect of not even allowing the rig’s negative terminal to serve as a (poor) ground via the vehicle’s neg-ground system. My initial results were similar to yours with varying SWR. My solution was less elegant than yours but electrically the same. On 30/40m, I usually put out a 33-foot wire as additional RF ground if the parking area allows it.

    73,
    John AE5X

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